In a fiber-optic system, it is sometimes necessary for a beam of light to emerge from a source fiber into free space and to later enter a destination fiber. To accomplish this, the beam of light that emerges from the source fiber must be guided across the free space so that as much light as possible from that beam enters the destination fiber. The extent to which light emerging from the source fiber fails to enter the destination fiber is referred to as the “insertion loss.”
The guiding of the beam of light across the free space is typically accomplished by lenses and mirrors. Since the source fiber is essentially a point source of light, the waves emerging from it are spherical waves. As they propagate through free space, these spherical waves tend to disperse the energy contained in the beam along a spherical wavefront. To reduce insertion loss arising from such dispersion, a collimating lens is typically placed in front of the source fiber.
The collimating lens transforms the spherical wavefronts that emerge from the source fiber into nearly planar wavefronts that propagate in a selected direction. The extent to which a collimating lens does so depends, to a great extent, on the position and orientation of the collimating lens relative to the fiber. If the collimating lens is not oriented correctly, the beam will point in the wrong direction. If the collimating lens is too far or too close to the source fiber, the beam will not have sufficiently planar wavefronts to remain collimated as it propagates toward the destination fiber.
Even small errors in the position and orientation of a collimating lens lead to unacceptable insertion losses. As a result, it is critical that the collimating lens be positioned correctly relative the source fiber. This is a difficult enough task in the case of a single collimating lens positioned over a single source fiber. It is made even more difficult when an array of collimating lenses is to be correctly positioned over a corresponding array of source fibers.